The usual compromise is to get something like a 20l day pack, which is big enough for your basic day kit and also doubles as a handy school or varsity book bag. Not a bad option, but not very individual.
On day hikes I prefer to eat well, carry lots of water, a bulky camera, map, 1st aid kit, and a few warm and waterproof alternatives in case the Cape mountains pull a fast one and it ends up snowing in mid summer. I gave up trying to fit all this kit into a traditional 20l day pack, mainly because I don't like carrying something that feels like a lead soccerball on straps.
My current day pack is an aged Karrimor Epic 35, which is a comfortable teardrop-shaped bag with plenty of space and some handy features like exterior mesh pockets. I use the Epic as hand baggage on overseas trips. It's big enough to fit a laptop inside, in the laptop briefcase, nogal.
My current favourite daypack is the K-Way Kilimanjaro, which my wife uses. It's a medium size daypack with a light frame to allow air to circulate between the pack and your back.